Heat exchangers are routinely employed within the automotive industry, such as in the form of radiators for cooling engine coolant, condensers and evaporators for use in air conditioning systems, and heaters. So as to efficiently maximize the amount of surface area available for transferring heat between the fluid within the heat exchanger and the environment, the design of the heat exchanger is typically of a tube-and-fin type containing a multitude of tubes which thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa. Increasingly, heat exchangers used in the automotive industry are being formed from aluminum alloys so as to help reduce the weight of automobiles.
Conventionally, heat exchangers are constructed using one of several methods. One such method utilizes mechanical expansion techniques and has been traditionally used for mass-producing radiators. Mechanical expansion techniques rely solely on the mechanical joining of the components of the heat exchanger, such as the joining of the tubes to the fins to ensure the integrity of the heat exchanger. Advantages to this method of assembly include good mechanical strength and avoidance of joining operations which require a furnace operation, while disadvantages include inferior thermal performance and relatively high weight.
To overcome the disadvantages of the mechanical expansion-type heat exchangers, heat exchangers are increasingly being formed by a brazing operation, wherein the individual components of the heat exchanger are permanently joined together with a brazing alloy. Generally, brazed heat exchangers are lower in weight and are better able to radiate heat as compared to mechanical expansion-type heat exchangers. An example of such a heat exchanger is referred to as the serpentine tube-and-center (STC) type, which involves one or more serpentine-shaped tubes which traverse the heat exchanger in a circuitous manner. The serpentine-shaped tubes are brazed to a number of high surface area finned centers with an inlet and outlet being located at opposite ends of the tube or tubes.
Another type of heat exchanger involves a number of parallel tubes which are brazed to and between a pair of headers, wherein finned centers are brazed between each adjacent pair of tubes. This type of heat exchanger is referred to as the headered tube-and-center (HTC) type. Conventionally, headered tube-and-center type heat exchangers have been constructed by inserting the parallel tubes into apertures formed in each of an opposing pair of headers. A finned center is then positioned between each adjacent pair of parallel tubes. Vessel-like members are placed at each header to form tanks therewith which are in fluidic communication with the tubes through the apertures. The tanks include ports which serve as an inlet and outlet to the heat exchanger. The above individual components are fixtured together before undergoing a furnace brazing operation that permanently joins the components to form the heat exchanger assembly.
One brazing technique which has become accepted by the automotive industry involves an inert atmosphere furnace operation. To destroy and remove the aluminum oxide layer on the aluminum alloy headers, tubes, finned centers and tanks, the assembly or its individual components are generally sprayed with or dipped into a flux mixture to enhance the brazeability of the brazing alloy prior to brazing. A conventional flux mixture consists of about 15 to about 25 volume percent flux solids suspended in water. The assembly is then dried to evaporate the water, leaving only the powdery flux solids on all of the external surface of the assembly.
A satisfactory type of flux for use with these aluminum alloys has been potassium fluoaluminate complexes, as disclosed in U.S. Pat. Nos. 3,951,328 and 3,971,501 to Wallace et al and Cooke, respectively. However because the flux is only introduced externally to the assembly (through dipping or spraying), the internal joints of the assembly, particularly the tube-to-header joints, do not directly benefit from the application of the flux, as evidenced by the high percentage of defective leaky assemblies, which at times may be as high as about ten to thirty percent. In addition, defective assemblies must individually undergo a costly repair, which increases the effective cost of producing the heat exchangers.
From the above, it is apparent that the brazing operation for a headered tube-and-center heat exchanger is complicated by the numerous brazements required for each tube, each of which must be brazed to both headers and its corresponding finned centers during a single brazing operation. Generally, the brazements are achieved by employing an aluminum alloy brazing stock material to form the headers and the finned centers. The aluminum alloy brazing stock material consists, for example, of an appropriate aluminum alloy core which has been clad on at least one side with an aluminum-based brazing alloy. Generally, the brazing alloy has been provided on both surfaces of the finned centers and on only the external side of the header, i.e., the side through which the tubes are inserted.
Typically, the cladding layers are an aluminum-silicon eutectic brazing alloy, such as AA 4045, AA 4047 and AA 4343 aluminum alloys (AA being the designation given by the Aluminum Association), which is characterized by a melting point which is lower than the core aluminum alloy, which is often AA 3003. Therefore, the brazing operation involves raising the temperature of the assembly such that only the clad layers of brazing alloy melt during the brazing operation. The brazing alloy then flows toward the desired joint regions and, upon cooling, solidify to form the brazements.
Conventionally, it is known to provide the brazing alloy as 1) a foil which is brazed to the extruded tubes of a serpentine tube-and-center type heat exchanger, 2) a molten coating which is deposited on the extruded tubes, or 3) a liner on an ingot which is hot milled to produce a silicon-clad aluminum alloy foil used to form the finned centers and headers of a headered tube-and-center type heat exchanger or finned centers of a serpentine tube-and-center type heat exchanger.
A shortcoming of the first two above-described processes, the brazed foil and molten coating processes, is that there are two fluxing operations required: the first to adhere the brazing alloy to the tube's aluminum alloy core, and a second to braze the tubes to the finned centers during the braze furnace operation. The need for two fluxing operations is disadvantageous in that the additional flux, its application, removal and the necessary effluent control procedures required to treat the waste water generated by flux removal, all add costs to the final assembly. In addition, the conventional spray and dipping methods required result in the deposition of flux on surfaces of the heat exchanger components which do not serve as braze joint areas and thus do not require flux. The additional flux also aggravates the tendency for the flux to corrode the interior of the furnace, resulting in additional maintenance and repair of the furnace.
Another disadvantage with the brazed foil and molten coating processes is that the silicon within the brazing alloy tends to diffuse into the aluminum alloy core at the elevated temperatures required for the brazing operation. As a result, the corrosion resistance of the brazing alloy is reduced and, due to the reduced silicon content in the brazing alloy, the furnace temperatures required to melt the brazing alloy are higher.
In addition, a shortcoming of the above-described hot milled method is that silicon clad aluminum alloy center stock material which is less than about 0.004 inch thick is difficult to obtain commercially, therefore generally resulting in undesirable additional weight to the heat exchanger. Another shortcoming is that relatively few aluminum suppliers can provide clad aluminum, and then generally only in excessively large volumes, such as a minimum ingot weight of about 7000 pounds. This is particularly disadvantageous to low-volume manufacturers. Further, the silicon clad aluminum alloy centers are more difficult to machine, thereby significantly reducing tool life.
Lastly, the general practice of cladding the aluminum alloy core with an aluminum-silicon brazing alloy also tends to be disadvantageous in that the silicon content of the clad brazing alloy may vary significantly. For example, in the more commonly used brazing alloys, the silicon content can vary between about 9 and 11 weight percent for the AA 4045 alloy, between about 11 and 13 weight percent for the AA 4047 alloy, and between about 6.8 and 8.2 weight percent for the AA 4343 alloy (AA being the designation given by the Aluminum Association). For every one weight percent variation in silicon within the brazing alloy, the melt temperature of the brazing alloy can vary by about 10.degree. F. This variability in silicon content significantly complicates the process control for the subsequent furnace braze operation.
From the above, it is apparent that it would be desirable to provide a method for furnace brazing a heat exchanger that does not involve an aluminum alloy core which is conventionally clad with a brazing alloy. In addition, it would be desirable to limit the quantity of flux applied to those internal and external regions of the heat exchanger which serve as joining sites, particularly the tube-to-header joints and the tube-to-fin joints of a headered tube-and-center type heat exchanger.
It would also be desirable to use a minimal quantity of flux, wherein the flux has a sufficiently high viscosity to enable the flux to be selectively applied while remaining on the surface to which it is applied. However, the conventional flux mixture used in the spray and dipping techniques has a sufficiently low viscosity such that the flux mixture is incapable of being consistently deposited at a predetermined region. Also, after evaporation of the aqueous solvent, the flux has a particulate shape which does not adhere well to the surfaces of the heat exchanger. Subsequent handling and assembly of the heat exchanger causes sufficient agitation to shake loose a portion of the flux particulates from the heat exchanger surface.
Another shortcoming associated with the use of the conventional flux mixture is that during brazing, it is extremely important that the furnace atmosphere have a dewpoint of about 45.degree. F. or below and a free oxygen level of 100 parts per million (ppm) or less. A common approach has been to employ high purity cryogenic nitrogen. In a high dewpoint or high oxygen-containing atmosphere, a greater amount of oxidation of the aluminum occurs during the brazing cycle, thereby requiring greater quantities of flux. Therefore, with the conventional approach wherein the flux solids are suspended in an aqueous solution, all of the water must be removed prior to the brazing operation. This is difficult to consistently achieve in a production environment. In addition, entrapped moisture and oxygen inside the tanks and tubes of the condenser assembly also impede brazing, thereby requiring complete purging of the assembly just before the brazing operation, which is again costly and difficult to achieve.
Therefore it would be advantageous to provide a method for brazing serpentine tube-and-center type and particularly headered tube-and-center type heat exchangers, wherein both the brazing alloy and a minimum quantity of flux can be preapplied together to specific regions of the heat exchanger during assembly so as to enhance the brazing of the internal and external joints, specifically between the tubes and headers and between the tubes and finned centers, respectively. By improving the uniformity and consistency of the internal brazed joints, the number of defective leaky assemblies should be significantly reduced. In addition, it would also be advantageous if the flux were not suspended in an aqueous solution, so as to minimize the amount of moisture surrounding the assembly during the brazing operation, thereby optimizing the integrity of the brazing procedure.